This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
D2D communications are well-known and widely used in many existing wireless networks, e.g., ad hoc networks. Examples of D2D communications include communications in compliance with Bluetooth and several variants of Institute of Electrical and Electronic Engineers (IEEE) 802.11 standards suite, such as, Wireless Fidelity (WiFi) Direct. The systems of the above examples operate in an unlicensed spectrum.
Recently, in the third Generation Partnership Project (3GPP), D2D communications, as an underlay to cellular networks, has been proposed as an approach to take advantage of the proximity of communicating devices and at the same time to allow the devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communications share the same spectrum as a cellular system, for example by reserving some of cellular uplink resources for the D2D communications. Allocating a dedicated spectrum for D2D communications is a less likely alternative because the spectrum is a scarce resource. Sharing, especially dynamic sharing between D2D services and cellular services is more flexible and may provide higher spectrum efficiency.
D2D communications may be ad hoc or network assisted. For example, a cellular network may assist a D2D connection by establishing security for the D2D connection and/or partly or fully controlling the setup of the D2D connection (e.g. device/peer discovery and resource allocation). A cellular network may also assist D2D communications by controlling the interference environment. For example, if using a licensed operator's spectrum for D2D communications, a higher reliability can be provided than operating in an unlicensed spectrum. To assist the D2D connection, the network may also provide synchronization and/or partial or full Radio Resource Management (RRM), which may e.g. comprise time and/or frequency resource allocation for D2D communications.
For D2D communications, the selection of an access scheme or mode, in particular whether the access scheme or mode should be contention free (CF) or contention based (CB), is still under discussion.
The contention based access scheme involves provision of a non-UE specific resource pool specifically allocated for D2D communications. The resource pool is either preconfigured for a UE capable of D2D communications (a D2D UE for short) when the UE is out of coverage of a cellular network (which will be referred to as “out-of-coverage scenario”), or configured via e.g. system information block (SIB) signaling when the UE is in coverage of the cellular network (which will be referred to as “in-coverage scenario”). The D2D UE then autonomously selects a resource within the pool and transmits data with the selected resource. With such a contention based access scheme, a collision between D2D UEs may occur if a plurality of UEs wants to initiate D2D communications using the same resource within the resource pool.
The contention free access scheme means that a central control entity (e.g. an evolved Node B (eNB) in a Long Term Evolution (LTE) system) allocates resources for a D2D UE when the UE wants to transmit data via D2D communications. For example, if a UE wants to initiate a D2D communication, the UE may inquiry its serving base station about available resources for the D2D communication. Then, the serving base station may allocate the resources dedicated for the UE and inform the UE. Subsequently, the UE may initiate the D2D communication using the allocated resources. With the contention free access scheme, the collision between D2D UEs can be avoided. However, it will take some time on requesting the base station to schedule resources for the D2D communication, which may increase the transmission delay.
The contention based access scheme is the only suitable solution for D2D communications in the out-of-coverage scenario. However for the in-coverage scenario, either contention based or contention free communication access scheme is possible.
Enabling D2D communications in a cellular network poses several challenges. The first issue facing D2D communications in the in-coverage scenario is the coexistence between the D2D system consisted of D2D UEs and the legacy cellular (e.g., LTE) system. Since D2D communications use LTE resources, it is inefficient to allocate dedicated resources for the D2D communications. From operator's point of view, the LTE system is more important than the D2D system because the commercial value of the LTE system is larger than that of the D2D system. So how to control D2D communications such that the LTE system would not be affected is an issue to solve.
The second issue facing D2D communications in the in-coverage scenario is how to avoid the conflict or collision in the contention based access scheme, and when to utilize the contention based access scheme and when to utilize the contention free access scheme.